Strain gauge



April 23, 1935- l.. w. BAUER ET AL 1,999,075

I STRAIN GAUGE Filed March 11, 1952 mven s tornegs Patented Apr. 23,1935 reise STBAEN GAUGE Application March 11,

lil Claims.

This invention relates to a strain gauge adapted to measure the changesin shape in structural members when these members are put under load.Byan accurate measurement of the contraction or elongation of astructural member, it is possible to thereby measure the stress set upin the member under a predetermined'load.

One object of .the present invention is to provide a strain gauge whichis extremely accurate in measuring small changes in length and which maybe readily used by inexperienced operators with slight chance of error.Y

Another object of the present invention is Ato provide a strain gaugeembodying two parts movable relative-to one another due to a change inshape of the member being tested, the two parts being constrained tomove parallel to one another, and the relative movement thereof beingmultiplied within the gauge.

Another object of the present invention is to provide a strain gaugehaving tw`o points adapted to be engaged in holes in a specimen undertest, the points being so arranged that they are connected with aparallel motion device constraining the points to move parallel one tothe other, the motion thereof being amplified within the gauge apredetermined amount and amplified further in an indicating instrumentassociated with the strain gauge.

A further object of the present invention is to provide novel points foruse with strain gauges which are adapted to be engaged in holes in thespecimen under test, the pointsV being so constructed that a linecontact is obtained with the specimen, whereby relativelyY inexperiencedoperators may use the gauge and obtain accurate resuits, since thepoints may not be engaged in the specimen in various angular positionstochange the reading of the gauge. Y

A further object of the present invention is to provide a strain gaugeof the type wherein a change of length between two points engaged byrelatively movable elements of the gauge is reflected in the reading ofthe indicator of the 4" gauge, the engagement between the movable partof the gauge, and the indicator being so arranged that the indicatoraccurately reflects the gauge movement of the relatively movable elementin 0 all positions thereof, thus eliminating errors due to the relativeangularity of the parts. More specifically, this object is accomplishedby theprovision of a spherical surface on the movable part of the gaugedirectly engaging a smooth flat sur- F face on the plunger of theinstrument, whereby W all movements of the lever carrying the spherical1932, Serial No. 598,236

member are accurately reflected in the instrument.

Another object of this invention is to provide a strain gauge which isrugged in construction, simple in operation, and wherein all of theparts are housed in a dust-proof Vcasing such that accu-Y rate readingof the instrument may be obtained over long periods of use.

These and various other objects of the invention will lbe apparent fromthe following description and appended claims when taken in connectionwith the accompanying drawing wherein Figure 1 is a longitudinal sectionthrough the preferred embodiment ofthe invention.

Figure 2 is a detailed section taken on line 2 2 of Figure l. l

Figure 3 is an elevation of one of the gauge points engaged with a holein a test member.

Figure 4 is a sectional view of another embodiment of the invention.

Figure 5 is a front elevation of the modification shown in Figure 4 withthe cover in position.

Referring to the drawing wherein like characters indicate like parts,the strain gauge ci Figures 1 to 3 includes a substantially rectangularhollow casing i having an open side. This open side is adapted to beclosed by a flat cover secured in position by machine screws engaging inopenings 2 in corner lugs of the casing l. The casing I is preferablymilled from a solid piece of medium steel in the shape shown andtogether with the cover functions as an enclosing housing for themovable mechanism of the gauge. However, it will be understood that thehousing l may be assembled from a plurality of parts, although theunitary casing is preferred. Adjacent one end of the casing l a lug 3 isprovided with an accurately threaded opening to receive the fixed pointli.

This point, as well as the movable point hereinafter described is shownin detail in Figure 3, and includes a carefully threaded rod 5 which isadapted for engagement in the threaded hole in the'lug 3. The. body ofthe point l is provided with a hole 6 for reception of a rod forassembling or disassembling the points. The lower portion of the pointil has a conical portion 7 terminating at its lower end in anotherconical portion 8. It will be noted from Figure 3 that the cone point iscut at a less sharp angle than cone portion l, and the part 8 is attenedat its outer end. As seen in Figure 3, this construction of the pointsresults in a line contact with the hole in the member being tested. Thisline of contact is designated a-b in Figure 3 in which the test memberII has a usual counter-sunk hole therein for test purposes. The portion8 has a less sharp angle in order to accurately seat in the test hole,the portion l being blunt so as to shorten the tip and increase itsstrength. Due to the parallel motion of the two test points ashereinafter described, the points may not tilt or assume angularpositions, and the contact is always along line cir-b. It will be seentherefore that when the points areY engaged with openings and pressediirmly in position the engagement with the test member will always beupon the line a--b under repeated tests. This is an important feature ofthis invention since it enables repeated tests to be made even bydifferent operators and the engagement of the pin 4 with the holes inthe test member will always be along the line a-b so that accuratereadings of the gauge are always obtained. It will be understood thatboth of the points of the gauge are similarly constructed.

Adjacent its opposite end the casing I is provided with an opening I2elongated slightly in the longitudinal axis of the casing, and themovable point I3 passes therethrough and is threaded into an accuratelytapped hole in a movable member I4. Member I4 has a pin I5 upon which isfreely pivoted a link IS. At its upper end link I6 is also pivoted on apin I1 projecting from lug IS on the casing I. The pins I5 and I1 of thelink I6 are accurately fitted in holes in the link to prevent any lostmotion and to allow free swinging movement of the link.

Member I4 is provided with an accurately tapped hole to receive thethreaded end of a rod I9 which extends longitudinally of the casing. RodI9 is threaded at its opposite end into a bifurcated bracket member 2l,locknuts 22 serving to secure the rod in accurately adjusted position.The threaded ends of rod I9are preferably right and left hand threads tofacilitate assembly. The rod I 9 is preferably accurately machined frominvar steel which has a very low temperature coeflicient of expansion,so that changes in temperature have a negligible effect upon readings ofthe gauge. If desirable, certain nickel steels can be substituted forthe invar steel which have a slight negative temperature coeilcient ofexpansion.

The bifurcated bracket 2| is bent upwardly and passes on opposite sidesof a swinging arm 23. As seen by the detail of Figure 2, the arm23 hastwo conical openings drilled from opposite sides thereof and the bracket2l has threaded openings in alignment with these conical openings forthe reception of threaded pins 24. The pins 2li terminate in projectingpins 25 having rounded ends. By this construction a line-to-line contactis obtained between the conical holes in the bar 23 and the rounded endsof pins 25 so that the parts may pivot with respect to one another withsubstantially no friction. Any slight wear at these lines of engagementymay readily be taken up by adjusting the threaded pins 24.

A block 2B secures a at spring 21 to arm 23, the block 28 and spring 21being positioned in a recess in the end of arm 23. Spring 21 is alsosecured to a lug on the casing Iv by the clamp block 29, theblocks 28and 29 being held in position by machine screws. `From an inspection ofFigure l it will be seen that the upper edge of the arm 23 is closelyadjacent the lower edge of the corner lug of casing I and clamp plate29. By this construction the arm 23 always moves about a pivotal pointwhich is the same under all conditions of use. If the spring 21 werefree along a material portion of its length, the arm 23 might pivotabout various points along the length of the spring. However, by theconstruction as shown, the pivot point of arm 23 is always the same,thereby giving absolutely correct reading of the strain gauge.

At its opposite end the arm 23 is provided with a spherical opening forthe reception of a hardcned steel spherical ball 3l. The ball 3l engagesagainst an absolutely flat portion 32 of a plunger 33V of the indicatinginstrument 34. The plunger 33 is preferably provided with rack teeth onone edge for engagement with one of the train of gears 35 to transmitmotion of plunger 33 to the indicating pointer 36. Preferably plunger 33is biased downwardly by a spring 31 built into the instrument. 'Iheinstrument 34 is secured to the casing I by means of a bracket 38. Inorder to maintain the parts in set position when the instrument is notbeing used, the arm 23 and the casing I are provided with openings forthereception of a tapered pin 4I which may be readily inserted in theseopenings to hold the arm 23 and the other parts of the strain gauge inaccurate position during shipment.

The operation of the gauge as thus described is as follows: The specimenmember to be tested is carefully prepared by having small countersunkdrill holes made therein similar to those shown in Figure 3. The points4 are then engaged in the drill holes and a reading is taken under noload. Then the specimen is placed under load and the points 4 againengaged with.y

the test holes in order to accurately indicate Ythe expansion orcontraction of the specimen under the test load. Due to the parallelmotion linkage, the points 4 always remain parallel and therefore theline of contact a-b (Figure 3) is alwaysA the same and errors cannotcreep into the reading of the gaugerdue to inaccurate positioning of thegauge in the test holes. The movable point I3 takes up a new position inaccordance with the expansion or contraction of the specimen, The partsareconstrained to move in a parallel direction due to the parallelmotion linkage provided by the pivoted link I6 at one end of the gauge,and the pivotal connection between pin 24 and the flat spring 21 at theother end of the gauge. The resultant motion of the movable point I3 isreected in an oscillation of the bar 23 about the pivotal point of theleaf spring 21. This movement is amplied due to the long leverage of thebar 23. In the embodiment shown,k the distance between the pins I5 andI1 of link IS is exactly one inch and the distance between the pivotalpoint of the pin 24 and pivot point on spring 421 is also exactly oneinch, and the distance from the center of the ball 3| to the pivot onspring 21 is exactly six inches. Thus the gauge as shown will give anamplified movement in a ratio of'six to one of the movement of themovable point I3.

Due to the absolutely flat portion 32 of the plunger 33 engaging withthe spherical surface of the hardened steel ball 3l, an exact point ofengagement is always maintained. The exact movement of the point I3 isreflected in a six to one amplified form on vthe plunger 33 of theinstrument 34, and the amplification remains always the same even inextreme positions of the parts due to the spherical ball 3l. Theinstrument 34 may further amplify this movement, dependent upon the typeof instrument used. However, it will be noted that a definite andviously described.

predetermined `amplification is obtained lin the stra-in gauge itselfirrespective of the instrument associated therewith. f

It will be understood that the invention is not limited -in any respectto the instrument shown in connection therewith, since 'other forms ofindicating instruments` may be used to indicate the movements of thehardened ball 3|. It'will furthermore be understoodthatlthe dimensionsas given, namely one inch betweenpins I5 and H and between pins `24 andpoint 2l, and six inches between ball 3| -and point 21 are illustrativeonly and not restrictive. It is contemplated that gauges in accordancewithvthis invention can be manufactured in various sizes, some of thegauges being one or two .inches long, these short gauges reducing anypossible error due to buckling of the specimen under test. Theprinciples of the invention will be the same irrespective of the sizeofthe gauge manufactured.

Referring now to Figures 4 and 5 wherein a slightly modified form isillustrated, asimilar casing l is provided, and the movable points 4 andI3 are similar` in construction to those pre- The movableY point I3 isthreaded into an opening in a floating link 4I which is pivoted to astationary block 42 by a parallel motion linkageas follows. The link 4Iand block 42 have openings 43 therein in which are positioned bent leafsprings 44 held in position by screws 45. These springs 44 engageagainst pins 4B which seat in V-shaped notches 4'I in the link 4| andblock 42. Pivoted on the two left hand pins 46 is a link 48, and pivotedon the two right hand pins 46 is the end. of a bar 5|.

It will be seen from the construction as thus far described that themovable link 4I is restrained to move in a parallel direction due to theparallel linkage arrangement, whereby the pin I3 must therefore moveparallel to pin 4. The arm 5I extends longitudinally of the strain gaugeas seen in Figure 4 and is provided with a seat at its opposite end forthe reception of a hardened steel ball 52 similar to ball 3| of theprior modication. The ball 52 engages the flat end 53 of a rod 54 of theinstrument 55. The strain gauge casing I is provided with two upwardlyextending posts 56 having handles 5l secured thereto for using thestrain gauge. As seen in Figure 5 the open side of the casing I isadapted to be closed by a cover 58 held in position by screws 59 whichengage with the correspondingly threaded openings in the casing I.

This embodiment of the invention operates substantially in the samemanner as that previously described. The floating link 4I is constrainedto move in a parallel direction and this movement is reflected by anoscillating movement of the arm 5I and this oscillating movement isamplifled due to the long leverage provided by arm 5I. In the embodimentshown, this movement is amplified in a six to one ratio, that is, theball 52 multiplies the movement of oscillation of the linkage in a sixto one ratio. Preferably a spring 60 engages the lower side of arm 5|and a portion of the casing I to assist in returning the parts to zeroposition, and to balance the spring in the instrument 55.

In both of the forms of the invention herein illustrated, it will beapparent that the movable parts of the mechanism are constrained to movein a direct parallel motion and that the movement thereof is greatlyamplified by the construction of the movable mechanism itself.Furthermore all errors in reading due tothe shifting of the pointofrengagement between the movable mechanism and the plunger of theinstrument are' eliminatedV due to the ball engaging at all timesagainst a flat surface on the instrument. The entireY mechanism of thegauge is housed in a substantially dust `proof casing. The parts of thegauge are lig-ht in weight and yet sturdy in construction and Vonce theparts are accurately assembled', the gauge will accurately indicatechanges size of a test specimen over long periodsof use.

- In the embodimentl shown in Figures 4 and 5, the heat imparted to thestrain gauge by the hands ofthe operator is prevented from reaching theoperating parts thereof due tothe fact that the handles-5l are spacedfrom the casing. It will be understood that similar handles may be usedwith `the embodiment shown in Figures l The invention may be embodied inother specific forms without departing fromthe spirit or essentialcharacteristics thereof. The present embodiments are therefore to beconsidered 'in all respects as illustrative 'and not restrictive, thescope ofthe invention being indicated by the appended claims rather thanby the 4foregoing description, and all changes which come Vwithin themeaning and range of equivalency of the claims are therefore to beembraced therein.

What is claimed and desired to be secured by United States LettersPatent is:-

1. A strain gauge comprising a casing, two gauge points adapted to beengaged with the member to be tested, one of said gauge points beingfixed to said casing, a movable member within said casing carrying theother gauge point, a plurality of links pivoted to said casing and tosaid movable member and allowing the gauge point carried by said movablemember to move parallel to the xed gauge point, one of said links havingan extension, and an indicating instrument having a movable plungerengaging the extension of said link whereby movement of said gauge pointand said link is multiplied and indicated on said instrument.

2. The invention as defined in claim 1 wherein the engaging portions ofsaid plunger and the extension of said link comprise one substantiallyspherical surface and one flat surface.

3. The invention as defined in claim 1 wherein the extension on saidlink has a spherical surface engaging a flattened part on said plunger.

4. A strain gauge comprising a casing, two gauge points adapted to beengaged with the piece to be tested, one of said gauge points beingfixed to said casing, a movable member within the casing carrying theother gauge point, a link pivoted to said casing and said member, a rodsecured to said member at one end, a link pivotally secured to the otherend of said rod, means pivotally mounting said last-named link to saidcasing, said last-named link having an extended arm, and an indicatinginstrument for indicating movements of said extended arm.

5. The invention as defined in claim 4 wherein said means pivotallymounting said last-named link to said casing comprises a at spring.

6. A strain gauge comprising a casing, a gauge point xed to said casing,a movable member in said casing carrying a movable gauge point, a pairof parallel links pivoted to said member and said casing, one of saidlinks having an arm extending longitudinally within the casing, and anindicating instrument secured to said casing and having a plungerengaging the end of said arm.

7. The invention as defined in claim 6 wherein said movable member isprovided with V notches, and said links have substantially circular pinsengaged in said notches.

8. A strain gauge comprising a casing, two gauge points adapted to beengaged with the piece to be tested, said points having two adjacentconical surfaces of different angles with respect to the axis of thepoint, the surface having the lesser angle being adapted for engagementwith the edge of the recess in the member to be tested, one of saidpoints being fixed to said casing, a movable member within the casingcarrying the other point, a'link pivoted to said casing and said member,a rod secured to said member at one end, a link pivotally secured to theother end of said rod, means pivotally mounting said last named link tosaid casing, said last named link having an extended arm, and anindicating instrument for indicating the movement of said extended arm.v

9. A strain gauge comprising a casing, a gauge point fixed to saidcasing, a movable member in said casing carrying a movable point, saidpoints each having two adjacent conical surfaces of different angleswith respect to the axis of the point, the surface having the lesserangle being adapted for engagement with vthe edge of the recess inthememberfto be' tested, said .movable member being provided withV-notches,- a pair of parallel links pivoted to said casing and havingsubstantially circular pins pivotally engaged in said V-notches, one ofsaid links having an armextending longitudinally Within the casing, andan indicating instrument secured to said casing and having a plungerengaging theV end of said arm. x

10. A strain gauge comprising a casing,V two gauge points adapted to beengaged with the member to be tested, one of said gauge points beingxedto said casing, a movable member with slight temperature coefcient ofexpansion within said casing carrying the other gaugepoint, linkspivoted to said casing and to said movable member limiting the gaugepoint carried by said movable member to moveparallel tothe xed gaugepoint, one of said ylinks having an extension, andi an indicatinginstrument having a movable plunger engaging the extension of said linkwhereby movement of said gauge point and said link is multiplied andindicated on said instrument.

GRANT A. DE SHAZER. LOUIS W. BAUER.

